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The mass-balance way of making chemicals

With the help of its suppliers, Covestro is using the novel technique to produce renewable polycarbonate

by Alex Scott
April 25, 2021 | A version of this story appeared in Volume 99, Issue 15


Chemical companies are increasingly looking to the mass-balance concept as a method of offering chemicals made from renewable raw materials. It’s a low-cost, low-risk way to get into the renewables field, but it’s also a wonky idea that can be hard for consumers to grasp.

In mass-balance production, firms substitute a portion of their usual fossil fuel raw material with a renewable alternative, such as waste vegetable oil or pyrolysis oil made from waste plastic.

Because the fossil and renewable hydrocarbons intermingle, no resulting product is made solely from renewable raw materials. But in a sort of accounting maneuver, companies assign the renewable content to a portion of their output.

For example, if a company makes 100 plastic bottles using a raw material with 10% renewable content, it can claim that 10 of the bottles are 100% renewable under the mass-balance concept, even though in fact all 100 bottles have 10% renewable content.

The approach requires an independent auditor to certify the origin and quantity of renewable feedstock and to accurately account for the renewable quantity at each stage of the chemical production chain. The process is cumbersome, but companies still like the mass-balance approach because they can do it in their existing plants. The environmental benefits include a reduced carbon footprint and—in the case of pyrolysis oil—the diversion of plastics from the environment.

The concept is catching on. “The number of mass-balance-certified production sites and related certificates has doubled in each of the past 2 years to more than 800 today and continues to increase rapidly,” says Inna Knelsen, sustainability system manager for ISCC System, a German certification firm.

Earlier this year, Covestro and partners unveiled a mass-balance manufacturing supply chain for making polycarbonate, an engineering plastic. For now, the renewable raw material will be a modest amount of waste bio-oil. But one Covestro partner, the refining company Neste, has started to test pyrolysis oil made from plastic waste. Read on to learn what the process looks like.

Step 1

Raw materials are prepared


Step 2

Neste makes naphtha

Porvoo, Finland

Neste mixes up to 10 renewable biobased raw materials—sources include waste animal fats and used restaurant oils—to make an oil that it converts into naphtha at its biorefinery in Porvoo, Finland. Toward the end of 2020, Neste started to test a plastic-derived pyrolysis oil in its oil refinery in Porvoo. Neste aims to consume more than 1 million metric tons per year of plastic waste in its refinery by 2030. “A challenge that Neste and other refiners face is that the quality of pyrolysis oil is very dependent on the plastic feedstock that was used to produce the oil,” says Isabella Tonaco, a Neste vice president.

Naphtha produced from any of the three sources—fossil fuel, bio-oils, and plastic waste—is identical, according to Neste. In general, an independent auditor certifies how much renewable oil enters the process—an amount that determines how much of the naphtha a company can claim to be renewable. Neste sells its 100% renewable naphtha—which it has branded Neste RE—to Borealis and several other firms as a raw material for base chemicals.

Step 3

Borealis makes phenol

Porvoo, Finland

Borealis receives the renewable naphtha from Neste and uses it to make benzene, cumene, phenol, and other base chemicals. Borealis can assign 100% of the Neste RE it buys to phenol, but in practice, it assigns about one-third to phenol and the rest to other chemicals for the purpose of making other mass-balance renewable products.

“For us, renewables and plastic recycling are definitely a priority to support the journey to circularity,” says Thomas Van De Velde, a senior vice president at Borealis. “The limiting factor for renewable phenol is currently the limited availability of renewable naphtha. But it is increasing month by month.”

Step 4

Covestro makes polycarbonate

Krefeld, Germany

Covestro combines phenol—including mass-balance-certified phenol purchased from Borealis—with acetone to produce bisphenol A, which it then combines with phosgene, creating polycarbonate. This atomic dilution limits the renewable component of the polycarbonate to a theoretical maximum of 72%.

Covestro received its first shipment of certified renewable phenol from Borealis in October 2020. Covestro is experiencing “considerable interest” in mass-balanced products from customers in certain segments, says Jimena Ruesta, head of sustainable strategy for polycarbonate at Covestro. Nevertheless, “the market is still small and has to be developed,” she says.

Step 5

Plastics converters create products for consumers

Germany and beyond

Plastics-conversion companies melt polycarbonate pellets from Covestro into forms such as furniture and automotive dashboards with the intention of selling them at a premium. Covestro hopes that a product at the end of its useful life will be chemically recycled into pyrolysis oil once again.

“In the past year, we have seen several consumer brand owners want to be certified so they can prove their products were produced with sustainable raw materials,” ISCC’s Knelsen says. “One of the biggest challenges is to market mass balance without becoming too technical.”


This article was updated on May 24, 2021, to correct several mistakes. A Neste plant in Porvoo, Finland, is a refinery, not a cracker. Neste’s brand of renewable naphtha is Neste RE, not Naphtha RE. And Neste’s oil is made from 100% renewable raw materials and thus does not have to be independently mass-balance audited.



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